Modern printing rolls may be formed in a number of ways, including by overcoating a smooth metallic core with a metal layer, followed by applying an outer ceramic coating. The outer ceramic coating then may be engraved using a laser to form a desired cell pattern in the ceramic. Various printing rolls or sleeves exemplified by anilox rolls, engraved rolls, form rolls, meter rolls, knurled rolls, ink applicator rolls, sleeves, ink transfer rolls, and the like may utilize the present invention if they wear during use. Printing rolls and/or sleeves typically include a series of engraved cells or the like upon their outer surface. The cells are configured to receive and transfer colorants such as ink in the formation of an image or coloration on a substrate.
In the operation of flexographic printers, for example, ink from a reservoir is transferred to a roll, such as an anilox roll. The anilox roll then transfers the ink to a printing plate, which may be mounted to the surface of a print cylinder. The web or substrate is printed when the print cylinder and inked printing plate roll over the web, transferring the image to the web. This process may be repeated thousands of times.
The outer surface of printing rolls, such as an anilox rolls, may experience significant amounts of wear, which are the subject of this invention. As the outer surface wears, cells in the outer surface of the roll are worn away and their volumes are reduced. These cells, which sometimes form an inverted pyramid or a hexagon, supply a fixed quantity of ink that is passed to the web or substrate during printing, based on the volume of the cell.
As the roll wears, and the cells erode, a significant reduction in ink volume is transferred to the printing substrate by each cell. For example, a 20% reduction of depth in an inverted pyramid cell may lead to a 40-50% reduction in volume of ink transferred depending upon the screen count and cutting angle. This undesirably may result in a noticeable decrease in print density in the transferred image.
Printing personnel may compensate for such wear by adjusting the intensity of ink applied to the roll. In the past, as a roll is used, the amount of wear has been estimated, and the ink concentration applied to the roll was periodically adjusted to increase the intensity of the ink, thereby compensating for such wear. To determine the actual amount of wear, it has usually been required that the roll be removed from service, and examined with a magnifying apparatus, such as a microscope. The periodic compensation for roll wear to maintain a consistent image is a significant challenge in conducting such printing processes.
For example, it is often difficult to determine how much wear a particular roll has experienced. The degree of wear, however, is necessary information for ink adjustments. Furthermore, estimates of the ink concentration necessary to produce the desired image are not always accurate. Errors easily can be made in the amount of ink applied to the roll, sometimes resulting in an undesirable or inconsistent image or coloration applied to the substrate.
What is needed in the printing industry is an apparatus and method for accurately and quickly determining the amount of wear that has been experienced by a roll. An apparatus that provides to printing personnel updated or xe2x80x9creal timexe2x80x9d information regarding the amount of wear experienced by the outer surface of such rolls would be very desirable. Such an apparatus and method of use of same could enable more accurate and more convenient adjustment of the amount of colorant or ink applied, thereby improving the continuity of high quality, consistent printing. An apparatus or method that could supply wear data or information without requiring that the roll be dismounted and examined by magnification devices would be particularly useful.
Sometimes, the print machinery is out of alignment, resulting in uneven wear along the length of the roll, or a roll when provided is xe2x80x9cout of roundxe2x80x9d. It would also be important for printing personnel to be aware of such abnormalities which can lend to off quality production. Wear indicating apparatus and methods that could alert to uneven wear, out of round rolls and the like would be therefore quite useful.
In the present invention, a printing device for applying colorant to a substrate is provided. A roll having an outer circumferential surface with a plurality of transferring cells receives colorant from a reservoir to a substrate. A wear indicator mechanism is configured to detect wear and transfer the colorant of the outer circumferential surface of the roll. In some applications, the roll may be an anilox roll with a ceramic coating on its outer surface. In some applications, an outer sleeve is applied to the exterior surface.
In another embodiment of the invention, a system for determining the amount of wear that has been experienced by a roll or sleeve in a printing system is provided. In the system, a reservoir supplies colorant to the roll which is metered by a doctor blade, with respect to the plurality of transferring cells located on the outer surface of the roll. The transferring cells receive colorant from the reservoir and transfer the colorant to a substrate. One or more doctor blades wipe across the surface of the roll, metering colorant to the transferring cells. The doctor blades function as a dam or barrier for the colorant reservoir. The printing roll rotates while bearing against the doctor blade(s), and the portion of the outer circumferential surface on the roll that is engaged by a doctor blade defines a wear surface, and does wear during use.
A wear indicator mechanism or means provided on the outer circumferential surface of the roll measures the amount of wear experienced by the roll. In one embodiment, the wear indicator includes a plurality of indicator cells at various predetermined depths upon the outer circumferential surface of the roll. In other embodiments, a single indicator cell is employed. The indicator cells may be provided in alignment with a wear scale, which provides numerical or other indication of the amount of wear that has been experienced by the wear surface of a roll. However, other embodiments do not employ a scale or other indicia, but instead rely upon erasure of a textured surface to indicate the degree of wear upon a roll.
In yet another application of the invention, a method for detecting the amount of wear experienced by a printing roll is provided. The method includes providing a cylindrical roll having a wear-detection mechanism configured to detect depletion of the outer surface of the cylindrical roll. The roll is rotated, thereby transferring ink from a supply to a substrate. As the wear surface of the roll is depleted, the wear indicator mechanism makes it possible to observe the amount of roll wear.